It is known that very precise motion control can be achieved utilizing induction motors which are pulsed at high frequency, the state of each pulse (presence or absence) being controlled in response to the actual current of the preceding pulse, typically in each of three phases of the induction motor. A usual scenario is a DC source, such as an AC-to-DC converter, that provides power to each phase of the induction motor in alternating polarities by virtue of switches which take turns conducting (are alternately opened and closed in succeeding cycles), in a threephase, overlapped fashion. By causing the turn-on and turn-off of these switches to occur at points in the cycle where the voltage across the switches is zero, sufficient switching losses are avoided so as to render the system significantly more effective (advancing the efficiency from in the 70's of percent into the 90's of percent). With low losses on the switches, alternative switching devices may be utilized, such as insulated gate bipolar transistors. Zero voltage switching is achieved by using a resonant link between the converter and the inverter. The use of a resonant link eliminates turn-on losses and reduces turn-off losses by an order of magnitude. Achieving a repetitive occurrence of zero voltage to reduce losses requires that the voltage across the resonant tank return to zero after each cycle of oscillation. However, since there are finite losses (the Q is not infinite), successive cycles will consume more and more energy so that the DC bus does not return all the way to zero volts. To overcome this problem, in a system disclosed in Divan U.S. Pat. No. 4,864,483, energy is fed into the resonant tank circuit in every cycle by momentarily shorting the DC bus by turning on all bus switches during the zero voltage period. Shorting of the inverter DC bus feeds energy back into the resonant link circuit but causes the voltage across the bus to have a high peak value, which can approach double the normal DC voltage for the bus. To limit the voltage across the bus, a clamp circuit is provided. The clamp circuit includes a very large capacitor which is initially charged to the difference between the allowed bus voltage and the maximum voltage which may occur as a consequence of the resonant circuit. The clamp also includes an insulated gate bipolar transistor with an antiparallel diode across it. When the DC bus voltage exceeds the sum of the source voltage and the voltage across the clamp capacitor, the diode is forward biased and begins to conduct out of the tank circuit and into the clamp capacitor. Although the voltage across the capacitor will change somewhat due to the addition of charge thereto, with a suitably sized capacitor the voltage remains essentially constant as the conduction of the diode clamps the DC bus at the desired maximum voltage. The diode current, herein referred to as a negative current, decreases to zero and current then must flow in the opposite direction through the bipolar transistor of the clamp circuit. While current is flowing out of the tank circuit to the capacitor, that fact is sensed and the clamp transistor is turned on; because the diode is conducting, the turn-on for the clamp transistor is at zero volts and therefore zero power loss. The transistor is maintained conducting until there is sufficient energy in the tank inductor to ensure returning the DC bus to zero volts at the end of the current cycle, at which time it is turned off. The turn-on and turn-off is controlled by comparing current in the clamp circuit with a scaled function of the voltage across the clamp capacitor (as in FIG. 11 of said Divan patent).
In the prior art, it is known to sample the current feed in each phase of the induction motor as the voltage across that phase goes to zero, thereby to provide an indication of what the pulse state (on or off) should be during the next cycle. However, it has been found that circuits of the type described provide current motor indications which may be as much as several percent in error. In devices such as elevators which require extremely close control over the torque, and therefore the applied current, of the induction motor, current errors of several percent are not acceptable.
It has been found that a resonant link inverter control of the type described hereinbefore has a characteristic of sporadically failing to operate at all as a result of failure to maintain oscillatory voltage across the DC link.